Synthesis gas comprising hydrogen, carbon monoxide, and carbon dioxide is used for the synthesis of oxygenated organic compounds and also as a fuel gas for the generation of steam and electric power in combined cycle power generation systems. Synthesis gas generated from coal or heavy hydrocarbons typically contains sulfur compounds such as hydrogen sulfide and carbonyl sulfide which must be removed prior to use. A higher degree of sulfur removal is required for the use of synthesis gas as a chemical feedstock than as a fuel gas; this is usually the case for carbon dioxide removal as well. Carbon dioxide is also present, and removal of carbon dioxide to some degree often is required for chemical feedstocks but is required less often for fuel gas applications.
Sulfur compounds and carbon dioxide, defined as acid gases, can be removed from synthesis gas by well-known commercial processes which utilize physical absorption at elevated pressures in organic liquids such as alcohols and ethers, or which utilize chemical absorption at lower pressures by reactive liquids such as amines or inorganic alkaline solutions. Regeneration of physical absorbents is accomplished by heating, pressure reduction, or stripping with a gas lean in acid gas components, or combinations of these methods. Regeneration of chemical absorbents is usually accomplished by heating to reverse the absorption reactions and liberate the acid gas components.
The coproduction of oxygenated organic compounds and electric power from synthesis gas is a commercially attractive operation, particularly for synthesis gas produced from coal. Methanol is one organic compound of particular interest for use as a peak shaving fuel as well as a marketable product. Other organic compounds can be produced from synthesis gas via methanol with the coproduction of electric power.
Because the requirements for the removal of acid gases are more stringent for synthesis gas used as a chemical feedstock than for synthesis gas used as fuel, separate acid gas treating systems may be required in integrated plants producing organic chemicals and electric power from coal-based synthesis gas. There is a need for improved integrated acid gas removal methods for such applications which minimize capital cost, operating complexity, and energy consumption while providing appropriate low levels of acid gas contaminants in the synthesis feed gas and fuel gas. The invention disclosed in the specification below and defined in the claims which follow provides an improved integrated method for acid gas removal in such applications.